The disclosures herein relate generally to multiple fan systems. More particularly, the disclosures herein relate to series configuration multiple fan speed control systems for computers.
Multiple fans in a series configuration are often used in redundant fan systems. A stream of air from an upstream fan feeds into the inlet of a downstream fan. Typically, both fans are selected to have the same specification and are intended to operate at the same speed. In such a series redundant fan system including two fans, both fans are operated at a part-speed level when both fans are operational. When one of the fans fails or is otherwise made inoperable, the other fan is switched to a higher speed operation, such as a full-speed level, ensuring that sufficient air delivery is provided by the single fan.
U.S. Pat. No. 5,687,079 discloses a computer having a plurality of electrically powered fans and an I/O port to produce a fan speed control signal for each fan. The computer includes a fan-speed-control device and measures air temperature and controls fan speed as a function of the air temperature. The fan control device varies each fan speed in response to the different control signals generated by the port such that the fans are operated at different speeds to limit acoustical noise associated with the beat frequency of the fans.
U.S. Pat. No. 5,546,272 discloses a cooling subsystem including a first fan and a second fan. A common plenum provides a shrouded pathway for air communication between the first and second fans. The first and second fans cooperate to provide an optimum rate of ambient air flow into the chassis to provide air exchange within the chassis. The air flow within the chassis is in a predetermined direction to provide directed cooling of a specified device within the chassis. Both of the fans are operated at a low-speed level to provide a minimum air flow and a minimum air exchange within the chassis. Upon failure of one of the fans, the other one of the fans is automatically switched from the low-speed level to a higher speed level.
U.S. Pat. 5,168,424 discloses an apparatus including one or more electrical devices. Each one of the electrical devices is located in a corresponding one of a plurality of positions defined in a first section of a housing. The system further includes at least one dual side entry fan positioned such that each side entry is opposite a device and a power supply unit. In operation, the fans provide forced air cooling of the devices and power supply. Each of the devices is selected from a plurality of possible types with each type presenting a different impedance to the flow of cooling air.
Even though the fans in a system are made to the same specification and are made by the same manufacturer, they will typically not operate at exactly the same speed. Variations in the operating speed of a fan can result from factors such as manufacturing tolerances, different applied voltages and fan circuit driver variations. The volumetric flow efficiency of two fans in a series fan system is adversely affected when the speeds of the fans are not maintained at a prescribed speed differential. For example, when the upstream fan operates at a speed level lower than the speed of the downstream fan, a pressure drop across the downstream fan results. The pressure drop causes reduced volumetric efficiency, decreasing the air flow output from the fan system relative to an optimized airflow output for the same fan system.
Accordingly, what is needed is a fan control circuit for use with series-configuration multiple fan systems that actively controls the speed of the fans to enhance the volumetric air flow efficiency of the fan system.
One embodiment, accordingly, provides a cost-effective fan control system that tracks the speed of a plurality of fans mounted in a series configuration and that actively adjusts the speed of at least one of the fans. To this end, a cooling system for an electronic device includes a redundant fan system having an in-series configuration of adjustable speed fans. A speed controller is coupled to the fan system including a first fan operated at a constant speed and a second fan operated at an adjusted speed to maintain a speed differential between the operating speed of the first fan and the operating speed of the second fan. A sensing device is connected to the fans and to the speed controller for maintaining the speed differential constant.
A principal advantage of this embodiment is that the volumetric flow efficiency of the fan system is improved.